More precisely the present invention is situated in the field of the coding of a sequence of digital images using motion compensation and temporal transforms by discrete wavelet transformation.
Certain algorithms in the field of the coding of a sequence of digital images propose solutions for associating points between two images.
These algorithms use motion compensated temporal filtering by discrete wavelet decomposition. These algorithms execute first of all a wavelet temporal transform between the images in the video image sequence and then spatially decompose the resulting temporal sub-bands. More precisely, the video image sequence is decomposed into two groups of images, the even images and the odd images, and a motion field is estimated between each even image and the closest odd image or images used during the wavelet temporal transformation. The even and odd images are compensated for motion with respect to each other iteratively in order to obtain temporal sub-bands. The iteration of this group creation and motion compensation process can be effected in order to generate different wavelet decomposition levels. The temporal images are subsequently filtered spatially by means of wavelet analysis filters.
At the end of the decomposition the result is a set of spatio-temporal sub-bands. The motion field and the spatio-temporal sub-bands are finally coded and transmitted in retrieval levels corresponding to the resolution levels targeted.
According to these algorithms the quantisation and entropic coding steps are performed following the decomposition. The coefficients of the high-frequency sub-bands, temporal or spatial, that are not directly used for the resolution of the requested retrieval level are eliminated.
When the motion between the different images of the video image sequence is a motion of the translation or rotation type, the high-frequency coefficients belonging to a higher resolution than that of the retrieval level can be omitted without artefacts appearing when the video image sequence is reconstructed.
When the motion between the various images in the video image sequence is a motion of the expansive or contractive type, the inventors of the present invention have noticed that eliminating the high-frequency coefficients belonging to a higher resolution than that of the restitution level creates artefacts when the image sequence is reconstructed.
An aim of the invention is to resolve the drawbacks of the prior art by proposing a method and device for coding and decoding a video image sequence that improve the video image sequence reconstruction, when contractive or expansive motions exist between the images in the video image sequence.